1. Field
The present disclosure relates to a system and method for determining whether the gear ratio of a continuously variable transmission (CVT) that uses pulleys is set to a maximum gear ratio at startup based on detected pressures within oil chambers coupled to the pulleys.
2. Description of the Related Art
Various types of continuously variable transmissions, or CVTs, for automobiles have been introduced to the marketplace. One of these CVTs utilizes variable-diameter pulleys including a primary, or drive, pulley assembly that is mechanically coupled to an engine and/or a motor generator. The pulleys also include a secondary, or driven, pulley assembly that is mechanically coupled to an output device, such as an axle. The two pulley assemblies are typically aligned and a drive belt is coupled to and received by both pulley assemblies so power can be transferred from the primary pulley assembly to the secondary pulley assembly. The pulley assemblies each include two pulleys, each of which has a conic shape with the apexes pointing towards each other, creating an hourglass shape. The relative location of the two pulleys of each pulley assembly can be controlled so that the pulleys are farther apart or closer together. For example, an oil chamber may be coupled to each of the pulley assemblies and pressure within the oil chambers can affect the position of the pulleys relative to each other. When the pulleys are farther apart, the effective radius of the belt about the pulley assembly is smaller than when the pulleys are closer together. Accordingly, the gear ratio may be increased by decreasing the radius of the primary pulley assembly and by increasing the radius of the secondary pulley assembly. Thus, the gear ratio of the CVT may be adjusted by varying the pressures within the oil chambers coupled to each pulley assembly.
It is desirable for a CVT of a vehicle to maintain oil pressure that provides sufficient force through the pulleys and to the belt such that relative rotation between the belt and pulleys is suppressed. This pressure may be different for various transmission unit designs. In many CVT type transmissions, a gear ratio may be designed to maintain sufficient force to the belt without applied oil pressure, and a gear ratio other than that which is designed to maintain sufficient force to the belt may allow relative rotation between the belt and the pulleys. This situation may arise during vehicle startup when oil pressure is not yet great enough to apply a sufficient force to the belt, causing undesirable wear of the pulleys and/or the drive belt. It is common that the maximum allowable gear ratio is designed to provide sufficient force to prevent relative rotation. If a control system of the vehicle is aware of whether the gear ratio is the maximum allowable gear ratio, the control system can utilize additional controls to protect the pulleys and/or the drive belt.
Traditionally, angular velocities detected at each pulley assembly are compared in order to determine gear ratios of the CVT. However, angular velocity sensors used in CVTs may not provide accurate data while the angular velocity of the pulley assembly is below a predetermined amount, such as 150 revolutions per minute (rpm). Accordingly, it is desirable to have systems and methods for determining whether a CVT is operating at a maximum gear ratio during and after engine startup.